The present invention is directed to an apparatus for isolating momentum of a motor from a drive means through which the motor drives a movable load to prevent the motor from damaging the drive means when there is a sudden change in speed of the movable load and the drive means. More particularly, the invention is directed to a motor which is formed integrally with a magnetic torque limiting coupling for protecting a drive means, such as a gear train normally driven by the motor, in the event of a sudden change in speed of the drive means.
When a motor is used to drive a movable device, such as an actuator, through a gear train, a sudden change in speed of the movable device as by jamming, for example, can result in significant damage to the gear train if the inertia or momentum of the motor's rotor is high enough such that it attempts to continue to rotate the gear train and movable device after they have been stopped. In the past, to solve this problem various different types of non-magnetic slip clutches have been developed and implemented in drive shafts between the motor and gear train.
An object of the present invention is to provide a novel solution to this problem, so as to prevent damage to the gear train or other drive means when the movable object is suddenly stopped, but the inertia of the motor causes it to continue to rotate. More particularly, an object of the present invention is to provide an improved apparatus for isolating momentum of a motor from a drive means through which the motor drives a movable load, to prevent the motor from damaging the drive means when there is a sudden change in speed of the movable load and the drive means.
An additional object of the invention is to provide an apparatus of the aforementioned type which is formed integrally with a motor which will operate normally, at least through its maximum operating torque, but which will protect the associated gear train or other drive means from damage in the event of a sudden change of speed of the load and/or the drive means. By integrating the drive means protection apparatus and the motor a smaller, lighter, low inertia and/low cost combination results.
These and other objects are attained by an apparatus according to the invention for isolating momentum of a motor from a drive means through which the motor drives a movable load to prevent the motor from damaging the drive means when there is a sudden change in speed of the movable load and drive means, causing a torque on the drive means above a maximum torque which may be safely applied to the drive means. The apparatus comprises a magnetic torque limiting coupling for transferring torque between the motor and the drive means. The magnetic torque limiting coupling has a magnetic strength which exceeds the maximum sustained torque which the motor develops during normal operation but which permits slippage to occur in the coupling in the drive between the motor and the drive means, to prevent a torque above the maximum safe torque from being applied to the drive means by the motor to thereby protect the drive means from damage by the motor in the event of a sudden change in speed of the movable load and the drive means.
According to a disclosed, preferred embodiment of the invention, the magnetic torque limiting coupling is formed integrally with a motor having a stator assembly, and a rotor and an output shaft concentrically disposed within a bore in the stator assembly. The magnetic torque limiting coupling operates to maintain the relative positions of the rotor and output shaft when the rotor is rotated, and transmits up to a predetermined amount of torque between the rotor and output shaft. The coupling is also operative to allow the output shaft and rotor to slip relative to one another when the torque demand exceeds the predetermined amount of torque. According to the invention, the predetermined amount of torque of the coupling exceeds the maximum sustained torque which the motor develops during normal operation but is less than or equal to a maximum safe torque which may be applied to a drive means to be driven by the motor to thereby prevent momentum of the motor from damaging the drive means, such as a gear train, coupled to the output shaft of the motor in the event of a sudden change of speed of the drive means.
A motor having an integral magnetic torque limiting coupling is known from U.S. Pat. No. 3,320,448. However, the coupling in such a motor allows slippage to occur at a torque limit which interrupts the normal operation of the motor, to avoid generating a great deal of heat, which may either cause damage to the winding of the motor or burn the motor out. Magnetic couplings are also shown in U.S. Pat. Nos. 4,381,466 and 4,486,176.
The disclosed coupling of the invention comprises first and second rotatable members which are located in spaced relationship and rotatably coupled to one another by magnetic means. The first and second members are arranged in telescoped relation with a plurality of permanent magnets being arranged on each of the members, with the magnets on one member being in opposed, spaced relationship to the magnets on the other member for magnetically coupling the members together to maintain their relative positions during rotation, and to transmit torque through the coupling.
The first member of the coupling is a hollow motor output shaft of the motor. The second member is a coupling output shaft of the motor and is telescoped within the second member. The hollow output shaft of the motor is rotatably connected with the motor rotor. The drive means in the disclosed embodiment includes a gear train which is protected from damage according to the invention.
According to a further feature of the invention the rotor of the motor is supported for rotation by journal extensions which extend outwardly beyond each end of the rotor for cooperation with bearing means of the motor, whereby the length of the magnetic coupling can be the same or substantially the same as the rotor to attain the necessary magnetic strength. The journal extensions are formed on the ends of the hollow motor output shaft of the motor in the disclosed embodiment.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, one preferred embodiment in accordance with the present invention.